Summary of work: Osteoarthritis (OA) is the most common form of arthritis in the elderly, and is a major cause of activity limitation, physical disability and health services utilization in the elderly. In the ongoing studies of OA in the Baltimore Longitudinal Study of Aging (BLSA), we have looked at the prevalence, incidence and progression of OA of the hand and knee, risk factors associated with having or developing the disease, as well as associated genetic markers and familial clustering. Our most recent work has focused on the longitudinal analysis of knee OA. Over a decade, approximately 14% of participants with both knees free of disease developed knee OA, while 44% with OA in one knee at baseline developed OA in the other knee. Other risk factors for incident knee OA were found to be older age, higher body mass index, and female gender. We also investigated the association of bone mineral density (BMD) and incident knee OA. After adjustment for the other known risk factors, we found that for every standard deviation (SD) above the mean radial (forearm) BMD in men, the risk of developing knee OA increases two-fold. In contrast, in women, higher BMD was associated with a reduced risk of incident knee OA. Other analyses underway are focusing on the incidence and progression of individual radiographic features of knee OA (e.g., joint space narrowing), and other potential risk factors for the disease (i.e., serum markers, change in BMD at different sites, and change in body composition over time).